My son went to NMMI a couple of years ago as a sponsored prep for USMA.
The personal cost depends upon a couple of things. First, are you sponsored by an academy? If so, do you know how much of a scholarship they will provide for you? For USMA the AOG scholarship was on a sliding scale based on the parents income. Second, NMMI offered a partial scholarship to the USMA preps. The other academy preps may also get a scholarship from NMMI but I'm not sure.
You can bring your own laptop if it meets the specs that NMMI requires. You would need to call NMMI and check with them on this. We rented the laptop for the year since my son did not have one. It wasn't very expensive, I think it was about $100 for the year.
One other thing...my son was in band at NMMI and this earned him some additional scholarship money. He also received some money from the ROTC program. You will want to look into the scholarships the school offers because you may qualify for some of them.
I'm at work right now, but tonight I could give you better numbers after I check my paperwork.